In recent years, as a semiconductor device is highly integrated, wiring of a circuit is miniaturized, and an inter-wiring distance is also becoming narrower. Consequently, it becomes necessary to planarize a surface of a semiconductor wafer that is a polishing target, and polishing by a polishing apparatus is performed as one means of a planarization method.
The polishing apparatus includes: a polishing table for holding a polishing pad for polishing the polishing target; and a top ring for holding the polishing target and pressing it against the polishing pad. The polishing table and the top ring are rotationally driven by a drive unit (for example, a motor), respectively. Liquid (slurry) containing a polishing agent is poured on the polishing pad, the polishing target held by the top ring is pressed against it, and thereby the polishing target is polished.
In the polishing apparatus, when polishing of the polishing target is insufficient, insulation between circuits cannot be taken, and short circuit might occur. In addition, in a case where excessive polishing is performed, there arises a problem, such as rise of a resistance value due to decrease in a cross-sectional area of a wiring, or the circuit itself not being formed due to complete removal of the wiring itself. Therefore, it is required to detect the most suitable polishing end point in the polishing apparatus.
As such a technology, there is an eddy current type end point detection sensor (hereinafter, referred to as an “eddy current sensor”) described in Japanese Patent Laid-Open No. 2012-135865. In the eddy current sensor, a solenoid-type or a spiral-type coil is used.
In recent years, in order to reduce a rate of defective products near an edge of a semiconductor wafer, there is a request of measuring a film thickness nearer the edge of the semiconductor wafer, and performing film-thickness control by in-situ closed loop control.
In addition, in top rings, there is included a top ring of an airbag head system utilizing a pneumatic pressure etc. An airbag head has a plurality of concentric airbags. In order to improve a resolution of unevenness of a surface of the semiconductor wafer by the eddy current sensor, and to perform film-thickness control in the airbags with a narrow width, there is a request of measuring a film thickness in a narrower range.
However, in the solenoid-type or the spiral-type coil, a magnetic flux is hard to be made thin, and there is a limit in measuring the film thickness in the narrow range.
An area of an eddy current (i.e. a spot diameter of the eddy current sensor) formed on a polishing surface of the semiconductor wafer by a conventional common eddy current sensor is not less than approximately 20 mm, a detection monitor region per spot is generally wide, and only an averaged film thickness of a wide range of region is obtained. Therefore, there is a limit in detection accuracy of presence/absence of a residual film and in accuracy of profile control, and particularly, variation in film thickness in an edge portion of a polished substrate cannot be dealt with. This is because in a conductive film, such as a copper film formed on the substrate being polished, the edge portion of the substrate serves as a boundary region, and a film thickness of a film formed at the edge portion is more easily changed compared with a film thickness of a film formed at the other position. In addition, it is generally difficult to detect a residual film with a width not more than 6 mm that remains in a part of the substrate, and the film to be normally polished may remain in a part of the substrate in some cases depending on fluctuations in a formation state of the film onto the substrate, or fluctuations in polishing conditions of the film, etc.
Note that one of reasons why the spot diameter of the eddy current sensor is wide is that a sensor coil diameter is large. In order to solve the above, it can be considered that the sensor coil diameter of the eddy current sensor is reduced, and that the detection monitor region of the film thickness is reduced. However, a distance from the eddy current sensor to the film in which the film thickness can be detected by the eddy current sensor has a correlation with the sensor coil diameter, and the distance becomes smaller as the sensor coil diameter is reduced. Since a polishing pad is present between the eddy current sensor and the substrate, a distance between the eddy current sensor and the substrate cannot be set to be not less than a thickness of the polishing pad. When the sensor coil diameter is reduced, the distance from the eddy current sensor to the film in which the film thickness can be detected by the eddy current sensor becomes smaller, it becomes difficult to form an eddy current on the substrate due to the thickness of the polishing pad, and thus detection of the film thickness becomes difficult.
Another reason why the spot diameter of the eddy current sensor is wide is that the magnetic flux widens since a distance between the semiconductor wafer and the eddy current sensor is large. When the same coil is used, the larger the distance between the semiconductor wafer and the eddy current sensor becomes, the wider the magnetic flux spreads, and the larger the spot diameter of the eddy current sensor becomes. The wider the magnetic flux spreads, the weaker the magnetic flux becomes, the weaker an eddy current formed on the semiconductor wafer becomes, and as a result, a sensor output becomes small. In order to accurately control a film thickness of the semiconductor wafer, an eddy current sensor that can measure a narrow range is desired.
The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a polishing apparatus that has an end point detection sensor capable of measuring a narrow range, and in which film thickness detection accuracy has been improved.